Right.
They will create the code, and they usually start an author’s thread in the “community created device types” section of this forum.
https://community.smartthings.com/c/projects-stories/community-created-device-types
But that section can be pretty hard to search, so the community wiki has a set of “quick browse lists” with one list for lighting, one list for motion sensors, one list for HVAC, etc. That way you can go right to the device type handlers that match what you’re looking for. 
Once you find one that you’re interested in trying, it’s basically a copy and paste process to add it to your own account. Here’s the FAQ for that, which also explains the concept a little more:
And if you have any questions, you can post them in the author thread and other people who are using that device type handler should be able to help.
Being able to add custom device type handlers is one of the best features of SmartThings, and the reason it can work with so many different devices. And the community is great, not only are people willing to share code, many are willing to help if you get a new device and need some modifications to make it work.